That gap is 4 percentage points narrower than a similar CNN poll of polls conducted March 26 through Wednesday. In that average, Clinton led the senator from Illinois 51 percent to 40 percent. Nine percent also were unsure then. Watch more on the polls »

The most recent poll of polls consists of three surveys: American Research Group (April 5-6), Muhlenberg College (March 27-April 2) and Quinnipiac (March 24-31). A sampling error for the poll of polls can not be calculated.

What's behind the shift?

"Obama has outspent Hillary Clinton three to one just on television advertising in Pennsylvania. He spent more than $3 million trying to get his name out and his message out to Hillary Clinton's $1 million," said Mark Preston, CNN's political editor.

The Illinois senator has also heavily benefited from the Service Employees International Union, which according to recently filed FEC reports has spent well over $700,000 on get-out-the-vote-efforts there.Watch more of Obama campaigning in Pennsylvania »